Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017
Income Tax [Line Items]
Deferred Income Tax Expense (Benefit)				$ 47,457	$ 48,457
Internal Revenue Service (IRS) | Latest Tax Year
Income Tax [Line Items]
Tax Year Closed				2011
NEW MEXICO | Latest Tax Year
Income Tax [Line Items]
Tax Year Closed				2011
TEXAS | Latest Tax Year
Income Tax [Line Items]
Tax Year Closed				2011
ARIZONA | Latest Tax Year
Income Tax [Line Items]
Tax Year Closed				2011
Change in Accounting Method Accounted for as Change in Estimate
Income Tax [Line Items]
Regulatory Asset, Amortization Period				15 years
Deferred Income Tax Expense (Benefit)	$ 100			$ 1,400		$ 2,200
Transmission and Distribution | TEXAS
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)	(1,500)	$ (900)		$ (1,700)
Depreciation | TEXAS
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)	$ 400
Settlement with Taxing Authority [Member] | ARIZONA
Income Tax [Line Items]
Unrecognized Tax Benefits, Period Increase (Decrease)			$ (300)